The invention relates to a CTD transversal filter with parallel inputs and evaluation circuits. The CTD arrangement comprises an output stage at which a filtered output signal can be tapped. Comparators are provided whose inputs are connected to counters supplied with clock pulses and connected to digital stores.
A transversal filter of this kind has already been proposed in the U.S. patent application Ser. No. 017,242 filed Mar. 5, 1979 (incorporated herein by reference) in which the digital signals input into the stores each determine the length of the opening times of the gate circuits which are individually assigned to the individual evaluation circuits. As the input of specific signal-dependent quantities of charge into the stages of the CTD arrangement is carried out according to the timing of the clock voltages supplied to the evaluation circuits, the opening time represents a gauge of the total quantity of signal-dependent charge which is input into a stage. Thus the opening time also determines the value of the evaluation coefficient assigned to an evaluation circuit. In the proposed transversal filter the stages of the CTD arrangement each comprise two or more electrodes beneath which the signal-dependent quantities of charge, having been input, are transported on in stepped fashion in the direction of the output stage of the CTD arrangement by means of clock voltages which are fed to the electrodes. However, at a specific frequency of these clock voltages for which the transmission of a given lowest signal frequency may not fall below, the aforementioned opening times are not always sufficient to achieve high evaluation coefficients with which a correspondingly large number of signal-dependent charges must be consecutively input into a stage.
This problem does not occur in other CTD transversal filters with parallel inputs which are disclosed for example in the Siemens Forschungs- und Entwichlungsberichten, Vol. 7 (1978) No. 3, pages 138 to 142, as in these the values of the evaluation coefficients is determined not by a corresponding number of periodic inputs of relatively small, constant quantities of charge, but by one single input of a quantity of charge whose size is determined by the area of one of the input gate electrodes contained in the evaluation circuit. However, with this method of coefficient formation, high coefficient values also necessitate a corresponding semiconductor area.